The present invention relates to a raster distortions correcting device to be used for picture tube of a television receiver and particularly to the top and bottom pichcushion distortions correcting circuits which is just suitable for picture tube having the aspherical face plate.
It is known that the pincushion distorsions generated at the top and bottom portions of raster scanning the face plate of picture tube of television receiver can almost be eliminated by modulating a vertical scanning current which flows through a deflection coil with an almost parabolic wave current or sine wave current. As the modulation circuit, a balanced modulator using active elements or a passive cirtcuit using saturable reactor is used in general. The device of this type is disclosed in the Japanese Patent Publication No. 28427/1981 and the Japanese Laid open Patent No. 75379/1983.
However, if a radius of curvature of face plate of picture tube is partially different, residual raster distortion is generated. A device which applies the signal current in resonance on the frequency equal to 2 times the horizontal scanning frequency (Japanese Patent Publication No. 43621/1976) or a device which switches the correcting signal corresponding to the switching of the display duration of video signal (Japanese laid-open patent No. 14025/1981) are proposed in view of further suppressing such residual distortion.
Such prior arts do not result in particular problems in case the face plate of picture tube has a constant radius of curvature or is formed as the flat surface. However, the television receivers having the aspherical face plate are widely put into practical use. Moreover, in the case of television receivers of the projection system, the phosphor screen of face plate of projection tube is formed in the aspherical convex shape in order to make short the optical distance for projection. The television receiver using the picture tube having the aspherical face plate as explained earlier provides a problem that residual distortion becomes very distinctive.
Outline of distortion correction of raster scanning the face plate of the picture tube having the aspherical face plate or the projection tube having the aspherical convex face plate (hereinafter, generally called the aspherical picture tube) is explained with reference to FIG. 5 and FIG. 6.
FIG. 5(c) indicates the modulation signal 103 obtained by modulating the parabolic wave signal current 104 of horizontal perid with the saw wave of vertical cycle. In case the top and bottom pincushion distortions of raster are corrected by this modulation signal 103, the raster 102 shows residual raster distortion called the Gull Wing as indicated in FIG. 5(b).
FIG. 5(d) indicates the sectional view of the face plate of picture tube. As will be understood from the relative positional relationship between an electron gun 108 and an aspherical phosphor surface 105, since the radius of curvature at the region 130 where the electron beam 106 and the beam 107 reach changes non-linearly in comparison with the picture tube having the flat or curved face plate, the raster corrected by the parabolic wave current 104 indicated in FIG. 5(e) has the Gull Wing type distortion.
FIG. 5(f) indicates the modulation signal 109 obtianed by modulating the sine wave current 110 of horizontal cycle with the saw wave of vertical cycle. When the top and bottom pincushion distortions are corrected by such modulation signal 109, the raster 111 shows the Napoleon hat type distortion as indicated in FIG. 5(g'). The pincushion distortions are generated in the inner scanning lines even if the top and bottom pincushion distortions of raster are perfectly corrected by the parabolic wave current 104 or sine wave current 110. Such raster distortion is indicated in FIG. 6.
Even when the distortions of top scanning line 112 and bottom scanning line 116 are corrected, the rather internal scanning lines 113, 115 shows the pincushion distortions due to the influence of the aspherical phosphor screen indicated in FIG. 5(d). Therefore, the pincushion distortions must be increased for the scanning lines 113 and 115.
The scanning line 114 at the center naturally does not contain the pinsuchion distortion.